X-ray topography is a well-known method for characterizing the microstructure of single crystals. It uses x-ray diffraction to measure defects and imperfections in crystal structures. A defect in crystalline structure affects how well the protein structure can be determined. Many of the basic macromolecules of organisms, e.g. proteins, DNA, RNA, can be crystallized. A defect in protein structure may affect the function of that protein, possibly resulting in disease.
There are currently many x-ray topography devices and methods of identifying defects in crystals using white beam or monochromatic x-ray topographs. The white beam technique (Laue geometry) provides rapid imaging but results in low effective resolution. Protein crystals tend to suffer severe and rapid degradation from the intensity of a white synchrotron beam. However, this technique is useful for rapidly screening samples. Monochromatic beam methods can provide more detailed information but require careful alignment and longer exposures. (Stojanoff, et al. (1996) Acta Cryst. A52:498-499; Stojanoff, et al. (1997) Acta Cryst. D53:588-595; Vetter, et al. (2002) Acta Cryst. D58:579-584; Boggon, et al. (2000) Acta Cryst. D56:868-880).
U.S. Pat. No. 6,385,289 (Kikuchi) teaches a device using two-dimensional information of x-ray intensities and a charge coupled device (CCD) camera with larger pixels (12 or 24 μm) than the present invention (less than 10 μm, preferably 8 μm) as described for x-ray topography. The device measures x-ray rocking curves using a movable x-ray detector which measures x-ray intensities diffracted off of the surface of a sample. Kikuchi's method diffracts x-rays off of a surface and measures the angle of the diffraction and the intensity of the x-rays when they strike the surface of an x-ray detector. In contrast, the present invention uses transmission geometry, where the x-rays are diffracted by passing through a sample.
U.S. Pat. No. 6,468,346 (Arnowitz et al.) teaches a method of improving crystal growth using magnetic fields or varying levels of gravity. Both earth and space-grown crystal samples were studied. The space-grown crystals were found to exhibit higher crystallographic perfection. X-ray topographic images are used to reveal defects in the crystals and to permit identification of the sets of conditions that produce crystals having the fewest defects. A method to generate topographic images is not taught.
Japanese patent 2000314708 teaches an x-ray topography apparatus with a movable stand, x-ray source and irradiation side slits which regulate x-rays from the x-ray source toward a specimen. The radiation side split is arranged between the specimen and a CCD sensor that detects x-rays from the specimen. A specimen support frame holds the specimen while it is measured.
U.S. Pat. No. 6,498,829 (Borgstahl et al.) teaches a method for mosaic spread analysis which uses super fine Φ-slicing data collection, unfocused monochromatic radiation and a suitable fast readable area detector such as a CCD. Random radiation events in the phosphor or optical taper intensities are removed from the diffraction data to create reflection profiles.
Most CCD systems presently used for crystallography have a fiber optic bundle in front of the CCD relative to the x-rays. The fiber optic bundle is generally tapered like a cone with the small end at the CCD. This provides a detecting area larger than the CCD chip. A phosphor is required because the fiber optic bundle is unable to transmit x-rays. The phosphor converts the x-rays into visible light. The fiber optic bundle also helps to protect the CCD from the damaging x-rays by absorbing the rays that are not converted by the phosphor into visible light. The fiber optic bundle has the disadvantage of reducing the spatial resolution of the CCD.
There is a need for a topography system that is low cost, rapid, quantitative and directly reads out to electronic media. Traditionally, single crystal topography was employed using x-ray film or nuclear emulsion plates. The currently available technology has some disadvantages, including long exposure times, slow sampling rate and chemical processes that can vary according to the protocol used, and the age of the reagents used. The present invention overcomes these problems by using a digital camera, giving the user real-time imaging, minimizing exposure times, and eliminating the variable chemical processes used by conventional methods.